冰山一角 (bīng shān yì jiǎo): tip of the iceberg

Investigators said this corruption case is just the tip of the iceberg of the entire corrupt network, with many more people involved still hidden behind the scenes.

Scientists found that the currently observed ocean plastic pollution may be just the tip of the iceberg; the amount of microplastics in the deep sea may far exceed our imagination.

He thought his wife was just occasionally in a bad mood, and only when she asked for a divorce did he realize that what he had seen was merely the tip of the iceberg of their marital problems.

The police chief admitted that the recently cracked theft gang case is just the tip of the iceberg; there are still multiple similar criminal organizations operating throughout the city.